1. Field of the Invention
The present invention relates to a method of using the Host Identity Protocol (HIP) to at least partially secure communications between two hosts, one of the two hosts being HIP enabled and the other not.
2. Description of the Related Art
When the Internet was originally devised, hosts were fixed in location and there was implicit trust between users despite the lack of real security or host identification protocols, and this situation continued even upon wider uptake and use of the technology. There was little need to consider techniques for dealing with host mobility since computers were relatively bulky and immobile.
With the revolution in telecommunications and computer industry in the early 1990's, smaller communication equipment and computers became more widely available and the invention of the World Wide Web, and all the services that emerged with it, finally made the Internet attractive for the average person. The combination of increasing usage of the network and mobile telecommunications created the need for secure mobility management in the Internet.
The increasing number of involved parties, and the monetary transactions that were needed for certain services, also created a need for added application level security. Currently, the most widely used encryption protocols, for example SSL/TLS, are running within the upper network layers, for example TCP.
Taking into account the above mobility management and security issues, the Mobile IP standard (C. Perkins, “IP Mobility Support for IPv4”, RFC 3220, IETF, 2002) and the Mobile IPv6 standard (D. Johnson, C. Perkins, J. Arkko, “Mobility Support in IPv6”, RFC3775, IETF, 2004) have been introduced. Together these specifications are planned to provide mobility support for the next generation Internet. Security work is developing in the form of IPsec, and related activities, such as various key exchange protocols, with the aim being to provide security in the IP layer. However, experience has shown that it is fairly hard to reach combined security and mobility using the current standards.
An IP address describes a topological location of a node in the network. The IP address is used to route the packet from the source node to the destination. At the same time the IP address is also used to identify the node, providing two different functions in one entity. This is akin to a person responding with their home address when asked who they are. When mobility is also considered, the situation becomes even more complicated: since IP addresses act as host identifiers in this scheme, they must not be changed; however, since IP addresses also describe topological locations, they must necessarily change when a host changes its location in the network. Clearly, it is impossible to achieve both stability and dynamic changes at the same time.
In the case of Mobile IP, the solution is to use a fixed home location providing a “home address” for the node. The home address both identifies the node and provides a stable location for it when it is at home. The current location information is available in the form of a care-of address, which is used for routing purposes when the node is away from home.
Another solution to the problem is to separate the identification and location functions from each other, and this is the approach taken in the Host Identity Protocol (HIP) proposal (R. Moskowitz, P. Nikander, P. Jokela, “Host Identity Protocol”, Internet Draft, work in progress, draft-ietf-hip-base-01, IETF, 2004). HIP separates the location and identity roles of IP addresses by introducing a new name-space, the Host Identity (HI). In HIP, the Host Identity is basically a public cryptographic key of a public-private key-pair, and is generated from and linked to the private key. The public key identifies the party that holds the only copy of the private key. A host possessing the private key of the key-pair can directly prove that it “owns” the public key that is used to identify it in the network. The separation also provides a means to handle mobility and multi-homing in a secure way.
HIP is discussed in more detail below, but is not the only proposal based around the idea of location and identity separation. FARA (D. Clark, R. Braden, A. Falk, V. Pingali, “FARA: Reorganizing the Addressing Architecture”. ACM SIGCOMM 2003 Workshops, Aug. 25 & 27, 2003) is a generalized model of ideas that provides a framework from which the actual architecture can be derived. FARA could make use of the HIP when the node identifications are verified, and consequently HIP could be a part of a particular FARA instantiation. The PeerNet proposal (J. Eriksson, M. Faloutsos, S. Krishnamurthy, “PeerNet: Pushing Peer-to-Peer Down the Stack”, IPTPS '03, Feb. 20-21, 2003) also discusses the location and identity separation. The Internet Indirection Infrastructure, I3 (I. Stoica, et. al., “Internet Indirection Infrastructure”, ACM SIGCOMM '02, Aug. 19-23, 2002) also defines a separation between the identity and routing information.
The Host Identity Protocol introduces a separation between the location and identity information at the IP layer. In addition to the separation, a protocol is defined to negotiate security associations (SAs) between HIP-enabled nodes.
With HIP, each host has one or more identities, which can be long-term or short-term, that can be used to identify it in the network. With HIP, an identifier is the public key of a public-private key pair. When the host possesses the private key, it can prove that it actually “owns” this identity that the public key represents; this is akin to showing an ID-card.
Each host can generate short-term keys to be used only for a short time. These are useful when it is not necessary for the node to be identified with the same identity later. For example, buying books from a bookstore may be a long-term relationship, while contacting a server once to collect user profiles may be considered to be a short-term action. In the latter case a short-term identity can be created to avoid more widespread dissemination of the long-term identity.
The HIP Host Identity (HI), being a public key, can be quite long and is therefore not practical in all situations. In HIP, the HI is represented with a 128-bit long Host Identity Tag (HIT) that is generated from the HI by hashing it. Thus, the HIT identifies a HI. Since the HIT is 128 bits long, it can be used for IPv6 applications directly as it is exactly the same length as IPv6 addresses.
Another representation of the Host Identities is the Local Scope Identifier (LSI), which is a 32-bit representation for the Host Identity. The purpose of the LSI is to facilitate using Host Identities in existing protocols and APIs. For example, since the LSI is the same length as an IPv4 address, it can be used for IPv4 applications directly. Although much of the remainder of this description will be based around the use of the longer HIT, it will be appreciated that the same or similar considerations apply to the alternative LSI representation.
When HIP is used, the upper layers, including the applications, no longer see the IP address. Instead, they see the HIT as the “address” of the destination host. The location information is hidden at a new layer, to be described below. The IP addresses no longer identify the nodes; they are only used for routing the packets in the network.
Applications are not typically interested in location information but do need to know the identity of their peers. The identity is represented by the HIT. This means that the IP address only has importance on lower layers where routing is concerned. The HITs, which the applications use, must be mapped to the corresponding IP addresses before any packets leave the host. This is achieved in a new Host Identity Layer as described below.
FIG. 1 of the accompanying drawings illustrates the various layers in HIP, comprising the standard transport layer 4, network layer 8 and link layer 10, with a process 2 communicating with the transport layer 4 below it. With HIP, a new Host Identity Layer 6 is disposed between the transport layer 4 and the network layer 8.
Locally, each HI and its associated HIT are mapped to the IP addresses of the node. When packets are leaving the host, the correct route is chosen (by whatever means) and corresponding IP addresses are put into the packet as the source and destination addresses. Each packet arriving from the upper layer contains the HIT of the peer as the destination address. The mapping between the HIT and the location information can be found at the HI layer 6. Hence, the destination address is converted to the mapped IP address, and the source HIT is converted to source IP address.
The mapping between a peer HIT and IP address can be retrieved in several ways, one of which being from a DNS server. The location information can be updated by the peer node any time. The update procedure will be discussed in more detail in the mobility management subsection.
HIP defines a base message exchange containing four messages, a four-way handshake, and this is used to create a security association (SA) between HIP-enabled hosts. During the message exchange, the Diffie-Hellman procedure is used to create a session key and to establish a pair of IPsec Encapsulating Security Payload (ESP) Security Associations (SAs) between the nodes.
FIG. 2 of the accompanying drawings illustrates the operation of the four-way handshake. The negotiating parties are referred to as the Initiator, starting the connection, and the Responder. The Initiator begins the negotiation by sending an I1 packet that contains the HITs of the nodes participating in the negotiation. The destination HIT may also be zeroed, if the Responder's HIT is not known by the Initiator.
When the Responder gets the I1 packet, it sends back an R1 packet that contains a puzzle to be solved by the Initiator. The protocol is designed so that the Initiator must do most of the calculation during the puzzle solving. This gives some protection against DoS attacks. The R1 initiates also the Diffie-Hellman procedure, containing the public key of the Responder together with the Diffie-Hellman parameters.
Once the R1 packet is received, the Initiator solves the puzzle and sends a response cookie in an I2 packet together with an IPsec SPI value and its encrypted public key to the Responder. The Responder verifies that the puzzle has been solved, authenticates the Initiator and creates the IPsec ESP SAs. The final R2 message contains the SPI value of the Responder.
The SAs between the hosts are bound to the Host Identities, represented by the HITs. However, the packets travelling in the network do not contain the actual HI information, but the arriving packet is identified and mapped to the correct SA using the Security Parameter Index (SPI) value in the IPsec header. FIG. 3 of the accompanying drawings shows the logical and actual packet structures when it travels in the network.
From the above it is clear that changing the location information in the packet does not create any problems for the IPsec processing. The packet is still correctly identified using the SPI. If, for some reason, the packet is routed to a wrong destination, the receiver is not able to open the packet as it does not have the correct key.
When an outgoing packet arrives at the HI layer from the above layer, the destination HIT is verified from the IPsec SADB. If an SA matching to the destination HIT is found, the packet is encrypted using the session key associated with the SA.
The HIT cannot be used to route the packet. Thus, the destination (and source) addresses must be changed to match the IP addresses of the nodes. These mappings are stored, as mentioned earlier, in the HI layer. After the addresses have been changed, the packet can be sent to the network where it is routed to the destination using the IP address information.
At the receiving host, the SPI value is used to find the correct SA form the IPsec SADB. If an entry is found, the IP addresses can be changed to corresponding HITs and the packet can be decrypted using the session key.
Mobility is defined to be the situation where a host moves while keeping its communication context active, or in other words the host changes its topological location, described by the IP address, while still maintaining all existing connections active. The processes running on the host do not see the mobility, except possibly if the experienced quality of service changes.
The mobile host can change the location inside one access network, between different access technologies, or even between different IP address realms, for example between the IPv4 and IPv6 networks. In HIP, the application doesn't notice the change in the IP address version. The HI layer hides the change completely from upper layers. Of course, the peer node must be able to handle the location update that changes the IP version and packets must be routable using some compatible address. If a node does not have both IPv4 and IPv6 connectivity, it may use a proxy node that performs the address version conversion and provides connectivity on behalf of the node.
Multi-homing refers to a situation where an end-point has several parallel communication paths that it can use. Usually multi-homing is a result of either the host having several network interfaces (end-host multi-homing) or due to a network between the host and the rest of the network having redundant paths (site multi-homing).
With HIP, the separation between the location and identity information makes it clear that the packet identification and routing can be cleanly separated from each other. The host receiving a packet identifies the sender by first getting the correct key and then decrypting the packet. Thus, the IP addresses that are in the packet are irrelevant.
A HIP Mobile Node (HMN), moving in the network, may change the point of attachment to the Internet constantly. When the connection point is changed, so does the IP address. This changed location information must be sent to the peer nodes, i.e. HIP Correspondent Nodes (HCN), and this is illustrated in FIG. 4 of the accompanying drawings. The same address can also be sent to a Forwarding Agent (FA) of the HMN, so that the HMN can be reached also via a more stable point. The DNS system is too slow to be used for constantly changing location information. Therefore, there must be a more stable address that can be used to contact the HMN. This address is the address provided by the FA.
The HIP Mobility and Multi-homing protocol (P. Nikander, J. Arkko, P. Jokela, “End-Host Mobility and Multihoming with Host Identity Protocol”, Internet Draft, work in progress, draft-ietf-hip-mm-00, IETF, 2004) defines a update (UPDATE) packet that carries the readdress (REA) parameter which contains the current IP address of the HMN. When the HMN changes location and IP address, it generates a UPDATE packet, signs the packet with the private key matching to the used HI, and sends the packet to the peer node and to the FA.
When the peer node receives a UPDATE packet, it must start an address verification process for the IP address that is included in the UPDATE packet. The address verification is needed to avoid accepting false updates from the HMN. It sends an update acknowledgement (UPDATE-ACK) packet to the address that was in the UPDATE packet. When the HMN receives an UPDATE-ACK that matches to the UPDATE sent earlier, it may start using the new IP address for sending data to HCN. After the peer node has received the first data packet from the new address, the address verification is completed and it can add the IP address as the location information of the HMN.
Because the HMN can move between networks using different IP address versions, the address received by the HCN may also be from a different address family than the previous address.
The HCN may support only one IP address version. In this case, the HCN must use some other proxy node that can be used for routing packets over to the other IP address version network.
A multi-homed HIP host, having multiple IP addresses configured on different interfaces connected to different access networks, has much more possibilities to handle the traffic towards a peer node. As it has multiple IP addresses presenting its current location in the network, it may want to tell all of these addresses to its peer nodes. To do so, the multi-homed HIP node creates a UPDATE packet that contains all the addresses that it is able to use towards that particular node. This set of addresses may contain all addresses it has, or some subset of these addresses. When the peer node receives the UPDATE packet with the multiple addresses, it must make address verification for each of these addresses to avoid possible false updates.
False, or non-routable, addresses in the UPDATE packet may be caused either because the HMN is a malicious node, it has an error in the stack implementation, or the HMN may be inside a network that uses private addresses that are not routable in the Internet. A multi-homed HIP node is able to use all of the available connections, but efficient usage of the connections requires a policy system that has knowledge of the underlying access networks and can control the usage of them. Such a policy system can use different kinds of information: user preferences, operator preferences, input from the network connections, such as QoS, and so on.
In order to start the HIP exchange with a mobile node, the initiator node needs to know how to reach the mobile node. Although Dynamic DNS could be used for this function for infrequently moving nodes, an alternative to using DNS in this fashion is to use the piece of static infrastructure introduced above, the Forwarding Agent (also referred to as a HIP rendezvous server). Instead of registering its current dynamic address with the DNS server, the mobile node registers the address(es) of its Forwarding Agent(s). The mobile node keeps the Forwarding Agent(s) continuously updated with its current IP address(es). A Forwarding Agent simply forwards the initial HIP packet from an initiator to the mobile node at its current location. All further packets flow between the initiator and the mobile node. There is typically very little activity on a Forwarding Agent, mainly address updates and initial HIP packet forwarding. Thus, one Forwarding Agent can support a large number of potential mobile nodes. The mobile nodes must trust the Forwarding Agent to properly maintain their HIT and IP address mappings. A Forwarding Agent can be used even for nodes that are fixed in location, since it is often the case that fixed nodes can change their IP address frequently, for example when it is allocated each time an Internet connection is set up by a Service Provider for that node.
The Forwarding Agent is also needed if both of the nodes are mobile and happen to move at the same time. In that case, the HIP readdress packets will cross each other in the network and never reach the peer node. To solve this situation, the nodes should remember the Forwarding Agent address, and re-send the HIP readdress packet to the Forwarding Agent if no reply is received.
The mobile node keeps its address current on the Forwarding Agent by setting up a HIP association with the Forwarding Agent and sending HIP update packets, containing readdress, to it. A Forwarding Agent will permit two mobile systems to use HIP without any extraneous infrastructure (in addition to the Forwarding Agent itself), including DNS if they have a method other than a DNS query to get each other's HI and HIT.
In the case of legacy equipment, a host may not be HIP-enabled, and the only option is to identify connections between hosts using IP addresses. This is not secure. The situation may be improved by locating a HIP proxy between the HIP-enabled host and the host which cannot use HIP. A typical scenario would be a small corporate LAN where the client terminals are not HIP-enabled. Traffic is routed to correspondent hosts (which are HIP-enabled) via the HIP proxy.
This arrangement is illustrated in FIG. 5 of the accompanying drawings. In FIG. 5, a legacy host 12 is shown communicating with a HIP-enabled node 14 (having the domain name “hip.foo.com”) via a HIP proxy 16. The legacy host 12 accesses the HIP proxy 16 over an access network 18 while the HIP proxy 16 accesses the HIP node 14 over the Internet 20. To partially secure the connection between the legacy host 12 and the HIP node 14, all communications between the HIP proxy 16 and the HIP node 14 are through a security Association set up between the HIP proxy 16 and the HIP node 14 in a similar way to that described above with reference to FIG. 3.
However, even before the Security Association 22 shown in FIG. 5 can be set up to enable communication between the legacy host 12 and the HIP node 14, a problem arises when the legacy host 12 tries to resolve the IP address of the HIP node 14 by sending a query to a DNS server 24-1 (and in turn DNS server 24-2) when the HIP node 14 is located behind a Forwarding Agent 26 as described above. The DNS server 24-1 will return the HIT of the HIP node 14 together with the IP address of the Forwarding Agent 26. As the legacy host 12 is not HIP enabled, it will disregard the HIT and start sending messages to the Forwarding Agent 26. Without the HIT, the Forwarding Agent 26 will not be able to resolve the destination address of these messages since it is most likely that several HIP nodes will use the same Forwarding Agent 26. Likewise, since the legacy host 12 discards the HIT and uses only the IP address of the HIP node 14 when initiating a connection, the HIP proxy 16 is unable to initiate HIP negotiation between itself and the HIP node 14 because it does not know the HIT of the HIP node 14. This problem is addressed in our PCT Application No. PCT/EP04/050129.
Other technical considerations arise when implementing HIP in third generation (3G) mobile telecommunications networks where not all of the User Equipments (UEs) in the 3G environment are HIP enabled. In this context, the Universal Mobile Telecommunications System (UMTS) is the 3G successor to the Global System for Mobile Communications (GSM). The most important evolutionary step of GSM towards UMTS is the General Packet Radio Service (GPRS). GPRS introduces packet switching into the GSM core network and allows direct access to packet data networks (PDNs). This enables high-data rate packet switched transmission well beyond the 64 kbps limit of ISDN through the GSM core network, which is a necessity for UMTS data transmission rates of up to 2 Mbps. GPRS is a prerequisite for the UMTS introduction.
It is desirable to provide at least some of the benefits of the Host Identity Protocol described above for communications between a legacy host operating within one network environment, such as UMTS or GPRS, and a HIP enabled host operating within another network environment, such as the Internet.